The invention relates to polyurethane foams, particularly to flexible polyurethane foams, and to a process for its preparation.
Polyurethane foams generally are prepared by reacting, in the presence of a blowing agent, a polyisocyanate with a polyether polyol or polyester polyol. The production of polyurethane foams is well known in the art. Flexible polyurethane foams are widely used for cushioning applications. Accordingly, the skilled artisans constantly seek ways for improving properties important for cushioning applications, such as load bearing properties and resilience, without impacting compression sets.
U.S. Patent Application Publication 2005/0261386 claims a “Latex-enhanced Polyurethane ‘Foam Cusioning’” by reaction of a polyol and an isocyanate including the use of a latex material to produce a ‘Foam cushioning’ having a CVR comfort value of more than 2.30 and up to about 2.60. This latex is vulcanizable and polymerizes during foam formation.
U.S. Pat. No. 3,755,211 discloses a process for preparing a flexible polyurethane foam with improved load carrying capacity by adding a polymer in the form of an unfoamed latex to an unfoamed mixture of a polyhydroxy compound, an organic polyisocyanate and water. The latex is an aqueous dispersion of a polymer, such as a polymethylmethacrylate latex, a polystyrene latex or a vinyl chloride-polyacrylate copolymer latex. The patent teaches that the glass transition temperature of the polymer must be greater than 50° C. to improve the load bearing property of the polyurethane foam. The polymer particles have a size of 200 to 800 angstroms (20 to 80 nanometers). The latex has a solid contents of 30 to 65 percent.
U.S. Pat. No. 5,312,847 discloses a polyurethane foam containing a particulate organic polymer having a glass transition temperature of less than 0° C. A preferred type of particulate organic polymer is a styrene/1,3-butadiene polymer. The particulate organic polymer is incorporated into the reaction mixture for producing the polyurethane foam in the form of a latex, preferably a latex wherein the discontinuous phase is water. The incorporation of the latex in the polyurethane formulation enhances the comfort aspect and SAG factor performance of the foam.
While the use of the latices disclosed in the above-mentioned U.S. patent publications improve some properties of flexible polyurethane foams, the use of these lattices is not unproblematic. The continuous water phase of the latex is a blowing agent for polyurethane foam. Accordingly, the amount of latex cannot be freely chosen to adjust the properties of the polyurethane foam. Also, commercially available polymer latices contain stabilizers to maintain the polymer particles in dispersion. The inventors of the present invention have found that such stabilizers may coagulate with components of the polyurethane composition, which has a detrimental effect on aging properties of the polyurethane foam, such as compression set.
Accordingly, it would be desirable to find a new way to further improve important properties of polyurethane foams, such as cushioning properties of flexible polyurethane foams.
Surprisingly, it has been found that the resilience of a flexible polyurethane foam can be increased by incorporating a redispersible polymer powder in the reaction mixture which is used to produce the flexible polyurethane foam.
Furthermore the redispersible polymer powder gives reduced foam load bearing at constant isocyanate index.